The invention relates to a disc brake for a commercial vehicle having a brake caliper which straddles a brake disc. The caliper is held on a stationary brake carrier such that it can be moved axially in relation to the brake disc. Brake pads are positioned in the caliper. During a braking operation, the brake pads can be pressed against the brake disc by way of a brake application device. A device for visual wear detection of brake disc wear and/or brake pad wear is provided.
In order to monitor the operating safety of a disc brake, there are wear systems which monitor the wear of the brake pads and/or of the brake disc, ensuring that necessary replacement of the relevant components takes place at the best possible point-in-time. Here, the state of wear of the brake pads should be established as accurately as possible.
For reasons of cost, replacement of the brake pads at an optimum point-in-time is required, the aim being to perform replacement only when the respective friction lining material of the brake pads has been worn down to a permissible minimum thickness. Earlier replacement would conflict with the constant striving to optimize operating costs, while replacing them too late would entail compromising the functional reliability of the disc brake overall.
Various design solutions are known for detecting the state of wear of the brake pads. Thus, there is a wear indicator in use in which a current-carrying cable is passed through the pad carrier plate. This cable is severed when the friction lining is worn. The severing is detectable from a signal in a signal transmitter usually arranged in the driver's cab of the motor vehicle.
In addition, the prior art includes wear indicators which use a rotary potentiometer to determine a travel distance of a wear adjusting device, which travel arises during adjustment of the brake pads to compensate for a wear-induced change in the release clearance, i.e. the distance between the brake pad and the brake disc. This can likewise be recognized, by the driver of the vehicle for example, from an indicator.
For direct visual detection of the state of wear of the brake pads, use is made of a device of the type in question, which has a spring-loaded pin which is guided in the brake caliper and by which a wear-induced adjustment of the brake caliper can be detected.
However, the implementation of a wear indicator of this kind is associated with considerable structural complexity, as is that of the other known wear indicators. For example, expensive machining of the brake caliper is required for guidance of the pin.
The electric wear indicators mentioned can only be implemented with a considerable outlay on equipment and, furthermore, do not guarantee absolutely reliable operation owing to the rough operating conditions in which commercial vehicles are used in many cases.
For visual detection of the state of wear of the brake pads and/or of the brake disc, there is a proposal in DE 36 12 166 C2 to provide the brake carrier with a marking, which is used in correspondence with a correspondingly modified pad carrier plate for wear detection.
In this case, the marks on the brake carrier are produced by corresponding shaping, in particular by U- or V-shaped recesses in the sense of notches.
However, such recesses and indeed other necessary shapings affect the strength of the brake carrier, wherein the notches, in particular, lead to a reduction in fatigue strength.
Common to all devices for wear detection is the fact that they are visible essentially only when the vehicle wheel is removed, and this conflicts with the desired simplicity of use of the device.
It is the underlying object of the invention to develop a disc brake of the type in question in such a way that simple, reliable and accurate indication of the wear of the brake pad is possible with little design and manufacturing outlay.
This object is achieved by a disc brake having a brake caliper, which straddles a brake disc, is held on a stationary brake carrier such that it can be moved axially in relation to the brake disc, and in which brake pads are positioned. During a braking operation, the brake pads can be pressed against the brake disc by way of a brake application device. A device for visual wear detection of brake disc wear and/or brake pad wear is provided. For wear detection, a comparison element, which extends in the direction of movement of the brake caliper and oppositely to the brake disc, is secured on the brake carrier. The comparison element is of strip-shaped design, with continuously planar side surfaces, and has at least one step on the edge side in the direction of movement of the brake caliper.
The strip-shaped comparison element, which, according to the invention, is secured on the brake carrier, preferably by latching, extends in the direction of movement of the brake caliper and oppositely to the brake disc and has at least one step on the edge side in the direction of movement of the brake caliper, is particularly simple and economical to produce and assemble.
This results in a considerable cost reduction as compared with the wear contacts mentioned above or with continuous wear sensing by the potentiometer described above.
Since, as mentioned, wearing parts, e.g. sliding contacts, are no longer used, there is also no abrasion, which otherwise proves disadvantageous in respect of environmental pollution.
Moreover, accurate indication of the state of wear of the brake pads and the brake disc is possible from the outside without removing the wheel rim. At the same time, this type of indication is unproblematic insofar as it does not require wearing parts, thus resulting in high reliability.
Detection of intermediate states of wear at predetermined intervals, e.g. at 50% intervals, can be achieved if a plurality of steps is provided, ensuring that an adequate estimate of the state of wear is possible at all times without problems.
According to the underlying principle, visual detectability of the state of wear of the brake pads and the brake disc is accomplished in correspondence with an edge of the brake caliper which is recognizably aligned with the corresponding step when a minimum permissible thickness of the friction lining is reached.
Since the comparison element is in the form of a separate component, it is not necessary to change the brake carrier or other components of the brake disc.
Another aspect of the invention produces the comparison element from a metal sheet, wherein at least one latching element, which engages in a latching opening in the brake carrier, is formed on a holding bracket.
Adjoining the holding bracket there are preferably two limbs, which are arranged parallel to and at a distance from one another and into which the steps are introduced. This embodiment is distinguished, in particular, by the fact that assembly can be performed manually in an extremely simple manner and does not require any additional assembly station.
If the comparison element is damaged or lost during operation, a replacement part can be secured without significant effort, given such an assembly.
One aspect which may be emphasized as particularly advantageous is the, in principle, universal applicability of the novel comparison element, which makes it possible to carry out retrofitting of a disc brake already mounted on the vehicle with a comparison element according to the invention.
As an alternative to the indicated selection of material for the comparison element, comprising sheet metal, it can also be produced from a plastic part or from a metal/plastic composite.
Instead of a brake caliper edge corresponding to the comparison element, it is also possible for other shapes to be used as references if they can be seen in correspondence with the comparison element in the manner described without removing the wheel rim.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.